Hypoxic training, often called high altitude training, has historically been assumed to be a foolproof way for endurance athletes to increase overall performance. Recent discoveries in athletic science, however, have started to dispel this myth. While hypoxic training can be a boon for athletes competing at high altitudes, training in oxygen rich environments may actually be more favorable for endurance athletes.

Hypoxia refers to a state in which the body receives lower amounts of oxygen than it’s accustomed to during workouts. The belief is that by training with limited amounts of oxygen, an athlete can acclimate their body to do more (power, endurance, etc.) with less oxygen.

In part, this theory has merit. Essentially, when the body performs rigorous activity with limited oxygen, the oxygen deficiency triggers the kidneys to secrete large doses of a hormone called erythropoietin. Erythropoietin signals to the body to create more red blood cells that, in turn, will carry more oxygen to its muscles and cells giving an athlete greater capacity.

While this physiological process does indeed take place, there are many drawbacks and limitations that ultimately make hypoxic training a good fit under certain circumstances. For one, training with less oxygen diminishes training intensity. Full acclimation or development of sufficient red blood cells typically does not occur for 15-20 days in a hypoxic environment. That means the body is training with starved oxygen for up to 3 weeks, resulting in a lower intensity workout.

Similarly, the body’s recovery time is slower in a low oxygen environment. Deprived of oxygen, muscles cannot restore and rebuild as quickly as they might otherwise. In some cases, immunity can even be impaired due to the presence of high amounts of the stress hormone, cortisol, that is often present during hypoxic trainings. In extreme cases, altitude sickness can even occur.

For all of these reasons, many athletes have difficulty maintaining or growing their muscle mass while training at high altitudes. In fact, some studies have shown that muscle mass actually shrinks when training in hypoxic states.

A final limitation to consider is that even after red blood cells have fully matured, the effects of altitude training wear off quickly once an athlete returns to lower altitudes. Typically, within 1-2 weeks of returning to lower altitude, all of the benefits will dissipate.

Rather than deprive the body of oxygen, scientists are discovering that athletes may receive greater benefits by training in high-oxygen environments; this is called hyperoxic training. Essentially, by flooding the muscles with oxygen, athletes can optimize their workouts, ultimately growing stronger and gaining endurance in a shorter amount of time. Unlike hypoxic training, muscle recovery time is also much quicker so that the next workout session can be equally as intense.

When considering all of these factors, training with low oxygen levels may only be ideal for athletes that are competing in high altitudes or for those that are unconcerned with speed, power and muscle mass (like ultra endurance athletes). Hyperoxic training, on the other hand, may be a better alternative for athletes wanting to build power, speed and muscle mass quickly.